Municipal solid waste (MSW) is an ever growing segment of our society that requires daily attention to maintain a healthy environment for our cities, towns, and the countryside. Past practices focused on burying the waste in the many landfills located outside population centers around the country. In today's society there are significant processes in place to reduce the amount of MSW going into these landfills to extend landfill life, reduce costs, and reduce adverse impacts of landfills on the environment. Significant among these processes are extraction and recycling of recyclable materials from MSW, production of compost from food waste and fiber streams, and in some cases incineration of portions of the MSW to reduce waste volume and to generate recoverable energy.
Today manufacturing of plastic bottles from polyethylene terephthalate (PET) for use in food packaging is widespread and growing. PET bottles are widely used worldwide to bottle both carbonated and still beverages. The bottles are favored by food and drink manufacturers for many reasons, among them PET bottles have clear walls so the contents are easily seen by the consumer, they are very tough and strong, and they have long shelf life for maintenance of carbonation. PET containers are well-suited for recycling. They are among the most expensive of packaging plastics and can provide substantial revenues for processes that recover and sale recyclables from the municipal waste stream. The recovery of used PET containers for use as a recycled material depends greatly upon the ability of automated sorting systems in use at today's materials recycling facilities (MRF's) to detect, identify, and segregate the used PET bottles from the municipal solid waste stream (MSW). There are several manufacturers of such automated sorting systems such as National Recovery Technologies LLC (Nashville, Tenn.), MSS, Inc. (Nashville, Tenn.), Titech (Akershus, Norway), Pellenc (Pertuis, France), and BT-Wolfgang Binder GmbH (Gleisdorf, Austria) most incorporating near infrared spectroscopic detection systems. Sorting systems from these vendors are in operation in industry for recovering PET plastics from MSW. However in recent years there has been a trend in industry to make the walls of PET beverage bottles thinner in order to reduce cost and lessen environmental impacts which causes problems for these sorting systems.
Thin walled PET containers are more difficult to detect and identify within the waste stream than are thicker walled containers due to the relatively small amount of PET material they incorporate. PET bottles for containing still water do not need to maintain carbonation and their walls in many cases have become very thin. PET bottles are a preferred package for still water bottling and particularly in individual serving sizes of about 16 oz and less, often referred to as single serving size. As it turns out many of these single serving sized thin walled PET bottles in addition to other PET bottles are discarded into the waste stream with their caps screwed on tightly and containing varying amounts of water or other liquid from just a trace to nearly full. It is not unusual to see bottles in the waste stream that are filled to ⅛, ¼, ½ and even more of capacity. These discarded PET bottles containing liquid are sometimes referred to in the recycling industry as “wet PET”. Water, and for that matter any liquid, will interfere with the infrared detection systems of modern automated sorters rendering these partially filled containers harder to detect. The general condition of the waste offers unique problems that need to be solved.
Near infrared (NIR) based sorting systems used in today's industry sense in the range of about 800 nm to 2400 nm and typically operate either in reflection mode or in transmission mode. In reflection mode broad band NIR radiation is often employed to be reflected off the surface of the material being analyzed (the sample—in this case it would be a packaging container). As the NIR radiation interacts with the sample surfaces energy to varying degrees is absorbed from the radiation at certain wavelengths, the amount of radiation absorbed and at which wavelengths being characteristic to the sample chemistry. The NIR radiation that is reflected is comprised of the original energy spectrum of broadband wavelengths of the impinging NIR radiation less energy absorbed at the characteristic wavelengths of the sample chemistry. These absorption features contained in the reflected radiation are detected by near infrared sensors positioned to receive the reflected NIR radiation and are analyzed by computerized detection algorithms to identify a sample according to its chemistry. For example NIR sorters can distinguish among the varying packaging polymers found in the municipal waste stream such as polyethylene terephthalate (PET), polyethylene (PE), polyvinylchloride (PVC), polypropylene (PP), polystyrene (PS), and others, and sort them accordingly. Processing PET polymer materials requires procedures that exploit these physical characteristics in order to allow for the separation and isolation which is desired. The need is great for enhancing existing processing methods.